The present invention is directed to a gradient coil design and manufacturing process for making self-shielded gradient coil sets for use in cylindrical bi-planar open magnetic resonance imaging (MRI).
Progress in MRI scanner design has taken two directions. The first has been towards higher field traditional magnet systems while the second has been towards low field open magnet systems. By “high field” magnet systems is meant those wherein the field strength is greater than 10,000 gauss and which require superconducting wire technology to generate the magnetic fields. “Low field” magnet systems are those in which the field strength is about 5,000 gauss and below and use permanent magnets or electrical coils. In general, the cost of the magnetic field producing material is a very strong function of the imaging volume. This is more so the case for open magnet structures than for cylindrical magnets. Therefore, image space management is crucial. In open MRI systems, the gradient and rf coils, which have to be placed within the imaging volume, take up significant space. In particular, an efficient and compact structure for stacking the gradient coil layers, which typically can take up to 20-30% of the image space in the vertical direction, is a very important goal. Moreover, the planar extent has to be limited as much as possible because the gradient coil must be contained within the magnet poles. The volume, size and weight of the magnets scale quadratically with the radius of the magnet. Thus this variation is the dominant factor that controls the cost of the magnet. Therefore, limiting the planar extent of the magnet and magnet poles should be a primary goal of any MRI system design.
In designing open, MRI systems the traditional approach has entailed using unshielded gradient coils and then trying to design a magnet system that would minimize the natural interactions between the gradient fields and magnets. Although the approach has had some success, it restricts the scanner from performing many MRI applications, particularly those requiring the use of bipolar current waveforms to drive the gradient coils due to eddy currents and residual gradient fields from conductive surfaces and magnet materials. In contrast, the present invention in which shielded gradient coils are used is based upon a systems approach that seeks to avoid all interactions between the magnet and the gradient and rf coils, thereby enabling all MRI applications to be possible.
It has also been discovered that the performance of open MRI systems can be further improved (and the cost lowered) by including active shims in the shielded gradient coils. The system can achieve up to two or more orders of magnitude improvement in magnetic field homogeneity by including 0th, 1st, 2nd and higher order active shims.
Furthermore, permanent magnets are heated differentially from unshielded gradient coils causing inhomogeneities and thermal drifts in magnet materials. It has been determined that adding cooling means in the gradient space can alleviate this significantly. Heretofore the cooling mechanisms had taken up too much space, so that the trade off had not been considered beneficial.
The present invention utilizes a gradient coil design that includes shims and cooling mechanisms located inside the gradient coil to achieve these improvements and allow a more compact open magnet design without sacrificing field homogeneity and performance.
Aside from physical constraints there are also imaging requirements on a gradient coil's performance that are met with the shielded gradient coils which allow the full complement of MRI applications. The use of shielded gradient coils that include active shims in an open MRI system has translated into a significant performance enhancement that combines stable homogeneity derived from self-shielding and additional active shim sets.
Unexpectedly, the present system has also been found to suppress the acoustic noise generated by the gradient coils. This is due to the inherent greater physical size of shielded gradient coils that combined with the opposing gradient fields of a shielded configuration helps to stiffen the gradient assembly and generate reduced torque effects.
The present invention is based upon a design methodology and a manufacturing process to make an open self-shielded gradient coil with the following features and benefits:                (1) ultra-fast switching;        (2) full 0th, 1st, and 2nd order active shims and the capacity to provide even 3rd, 4th, 5th or higher order active shims if needed;        (3) compact physical thickness and planar extent to maximize subject access space and minimize magnet pole face space;        (4) air or water cooling; and        (5) construction and mounting method to suppress acoustic noise generation.        
The method and manufacturing process described enhance open magnet MRI performance substantially while providing significant cost benefits due to the substantial reduction in the open magnet size.